The dichroic optical filter of the invention is particularly useful when embodied in a surgical operating microscope micromanipulator. Such a micromanipulator is an attachment to a surgical operating microscope which allows a surgeon to manipulate a high power laser beam (typically superimposed with a visible, coherent, aiming beam) while viewing a patient and the aiming beam through the microscope.
The principal optical components of a surgical operating microscope (with micromanipulator) are shown schematically in FIG. 1. The central element in the micromanipulator is beam combining optic 4 (sometimes denoted herein as "combiner" 4). Coherent beam source 2 (which may include one or more lasers) emits high power coherent beam 12 and visible, coherent aiming beam 14. Beam 12 is typically an infrared beam having wavelength 10.6 micrometers (from a CO.sub.2 laser). Aiming beam 14 is typically a visible beam from an HeNe laser having wavelength 0.6328 micrometers, although in alternative embodiments beam 14 can have any of a variety of other visible wavelengths (such as 0.543 micrometers). Beam 12 will sometimes be referred to herein as the "operating" or "treatment" beam. Operating beam 12 and aiming beam 14 are incident on patient 10 after they reflect from combiner 4.
Visible radiation 16 reflects from patient 10 and propagates through combiner 4 to microscope objective lenses 6 and 8. Two objective lenses 6 and 8 are shown to indicate that the microscope is binocular. Portion 14a of aiming beam 14 also reflects from patient 10 and propagates through combiner 4 to lenses 6 and 8. In this way, a surgeon may view the radiation transmitted through lenses 6 and 8 to determine the portion of patient 10 from which beam 14a has reflected.
Portion 12a of operating beam 12 reflects from patient 10, and then reflects from combiner 4 in a direction away from microscope objective lenses 6 and 8. In this way, combiner 4 prevents damage to the surgeon's eyes while the surgeon views radiation (14a and 16) transmitted through combiner 4.
In one conventional variation on the system of FIG. 1, combiner 4 is replaced by a substantially 100% reflective mirror that is mounted in a position offset from the path of visible radiation 16 from patient 10 to lenses 6 and 8. An important disadvantage of this type of conventional system is that it introduces parallax between the viewing light (reflected radiation 16) and the treatment radiation. This parallax makes it difficult or impossible for a surgeon to view and treat at the same time inside a restricted orifice of a patient's body (or through a hollow instrument inserted into such orifice).
In another type of conventional system, combiner 4 is replaced by a small, 100% reflective mirror. By symmetrically positioning such mirror between lenses 6 and 8 and patient 10, parallax is eliminated. However, the constraints on the size of the mirror in such a system render the system unsuitable for many applications. The mirror must be sufficiently small so as not to obstruct unduly the surgeon's view of the patient. Yet, the mirror must not be so small that diffraction effects prevent it from directing the treatment and aiming beams to a sufficiently small focal spot on the patient. Due to diffraction effects, the smallest spot achievable at the focus of the treatment and aiming beams is inversely proportional to the size of the reflecting mirror. Extremely small spot size is highly desirable for some forms of treatment, and yet cannot be achieved with this type of conventional system.
In yet another conventional system, combiner 4 of FIG. 1 is implemented as a dichroic filter, which transmits visible wavelengths (i.e., visible radiation 16 and visible aiming beam 14a) and reflects the treatment beam wavelength. With this approach it is possible to make the dichroic filter large (to achieve sufficiently small treatment beam spot size) and still keep the dichroic filter on the optical axis (for parallax control). One such conventional dichroic filter, suitable for use with a CO.sub.2 laser treatment beam having 10.6 micrometer wavelength, is filter 20 shown in FIG. 2.
Filter 20 is an "enhanced transmission" filter consisting of transparent glass substrate 20, thin dielectric layer 24 coated on substrate 20, thin gold layer 26 coated on layer 24, and thin dielectric layer 28 coated on gold layer 26. Gold layer 26 efficiently reflects 10.6 micrometer radiation.
The tendency of gold layer 26 to reflect visible radiation is partially overcome by dielectric layers 24 and 28, which produce a standing wave in the visible band, with an antinode at gold layer 26. The net effect is to enhance the transmission of visible radiation through filter 20.
Because coating layers 24, 26, and 28 would not efficiently reflect visible aiming beam 14 to the patient, it is conventional to include a small aluminized reflecting spot 29 (shown in FIG. 3) in the center of filter 20. Typically, filter 20 is mounted symmetrically with respect to the microscope objective lenses (6 and 8), so that spot 29 is symmetrically positioned relative to the objective lenses as shown in FIG. 3. However, if spot 29 is small enough not to interfere with the microscope user's view, it tends to produce an imperfect aiming spot in the field of view (due to the diffractive effect discussed above, and to loss of light). Furthermore, a typical spot 29 interferes with the microscope user's view of the patient, particularly when the microscope is operated at low magnifications.
For this reason, aluminized reflecting spot 29 and the aiming beam are sometimes omitted. Instead, a separate aiming spot is developed and projected into the microscope field of view as either a real or virtual image. However, it is difficult to keep such separate aiming spot aligned with the treatment beam.
Conventional enhanced transmission filter 20 (of FIGS. 2 and 3) has a number of additional serious limitations and disadvantages. For example, coatings 24, 26, and 28 attenuate a significant fraction of visible radiation incident thereon. Furthermore, gold does not adhere well to the usual dielectric materials employed as layers 24 and 28. Thus, coating layers 28 and 26 do not stand up well to the rugged environment of the operating room, and to subsequent cleaning.
The invention avoids the described limitations and disadvantages of conventional micromanipulator beam combining optics, by employing an oxide semiconductor coating (such as a layer of indium tin oxide) on a substrate (a substrate transparent to visible radiation), to reflect the treatment beam wavelength (or wavelengths) while efficiently transmitting visible wavelengths. Oxide semiconductor layers, such as layers of indium tin oxide ("ITO") have been used as transparent electrodes in electro-optical devices such as cathode ray and liquid crystal displays. However, until the present invention it had not been known to employ a transparent substrate with an oxide semiconductor coating as a dichroic filter, for such applications as use in a micromanipulator beam combining optic.